The Milky Way is much lighter than expected. It would be four to five times smaller than what scientists had calculated.
Hervé Poirier, editor-in-chief of the scientific magazine Epsiloon explains to us today that the Milky Way has finally been weighed. And the recent data, collected by the ESA’s Gaia satellite, surprised many physicists and researchers.
franceinfo: The surprise is that our Milky Way, also called Galaxy, is much lighter than expected. Explain to us…
Hervé Poirier: 409,000 billion billion billion billion billion kilos, the equivalent of 206 billion Sun: vhere, finally, is the mass of our Way Milky. And yes, it is 4 to 5 times less than previous evaluations.
How was this measured?
By an indirect method – we cannot put our Galaxy on a scale. The principle: the more massive a body is, the more the objects that gravitate around it have a high speed – otherwise they end up falling on him. Except that as much as it is quite easy to measure these rotations in other galaxies, it is complicated for ours.
Difficult to distinguish movements and distances, while we are immersed inside the Milky Way. But the Gaia telescope has patiently collected the trajectories of 1.8 billion stars over the past 10 years. Which now allows us to deduce precisely and with certainty the mass of our world.
But what should we think of this significantly reduced value?
Honestly, it’s weird. Our Galaxy is very light compared to the others. But there’s something even weirder. In all other spiral galaxies, the rotation speed of the stars is approximately constant, no matter how far they are from the center. This is what pushed astrophysicists, since the 1970s, to assume the existence of “dark matter”, invisible and massive, 6 times more abundant than classical matter, which forms a halo around galaxies.
However, this is not what happens in our good old Milky Way: they rotate less quickly on the outskirts. As if, here, there were three times less dark matter than elsewhere. Well, after all, why not. Our Milky Way could have had a particularly peaceful life, with very few collisions, which would explain these oddities.
But physicists don’t like that. They don’t like having to assume that we live in a singular place, and relying on models that are valid everywhere except here. They therefore begin to wonder if we were not mistaken in the measurements of other galaxies. Or if there’s nothing else wrong. It’s very exciting: we’ve never known our world so well. But we have never understood so little what is going on there…